How to display mirror reflection in viewport?
The viewport has several ways of displaying your scene. 

During modelling you might use Solid Shading, Material Shading,  Wireframe, or even GLSL (matcap) shading can be useful.

Reflectivity can only be seen (June 2015) in Rendered Viewport mode, this works for Cycles and Blender Internal engines, these options can give you a quick approximation of what the final render will look like.